Hard Turning a Clean Sharp Edge
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    Question Hard Turning a Clean Sharp Edge

    We are hard turning some circular knives (60 HRC), and are struggling to get a clean sharp edge, see attached pictures. We tried many different speeds, feeds, and depth of cuts. As well as using different CBN and ceramic cutting tips (V style, 0.8 rad), but whatever we do we can’t get the edge to clean up and not have any burrs.

    Have you got any suggestions on how we can get a clean sharp edge? Is there a tool or method we are missing? We are using a Doosan Lynx CNC lathe.

    Thanks

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    IMO that's a grinding job.

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    Yes but we are trying to turn it rather than grind it. Thanks

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    looks like some carpet knives I used to make for the automotive industry. I used a .032 radius ceramic insert for the final pass, dont remember the speed/feed, been years since I made them. I bet I was on the slow side, 300 sfm and .003 per rev would be my first guess. Sounds slow, but speed means heat and heat means you ruin the edge. I do remember that last, I used a fine, flat stone to clean up the edge, by holding it flat to the angle as it turned. They were sharp enough to cut you easily.
    I have done a lot of hard turning trim tools for plastic work and I have found that while you will not end up with as good a quality edge as grinding, you can turn/stone a working edge if you have a flat surface to rest your stone on so that you do not roll the edge.

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    Quote Originally Posted by Limy Sami View Post
    IMO that's a grinding job.
    This.

    What we used to do was grind and super-finish the primary bevel. We would hard-turn the secondary (if it had one) to maintain the primary bevel width (usually 1 mm).

    Looking at the grind on the reverse bevel, you do not want the ground finish going across the slitter. The grind direction should go radially. If this is a printing or paper converting slitter, this type of grind causes more dust and does not wear as well. The bottom knife is what turns the top slitter. The top slitter is canted at an angle and that type of grind does not work as well as one that is radial.

    If you are going to continue to try hard turning, make sure you are turning starting at the cutting edge and cutting toward the heel of the bevel. This will minimize the burr. You will still have to hone the edge. Depth of cut should be around .005" with maybe a finish pass at .002".

    We would sometimes hard-turn bottom knives depending on application. You could get a good edge since the bevel was only three degrees off vertical. We would ALWAYS grind and super-finish the top knives. If the bottom knives were carbide, then they were super-finished after grinding.

    You are not going to get the same edge quality by turning that you will get with proper grinding and super-finishing.

    Bill

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    Just an idea.

    Maybe turn the final cut to almost full radius, leaving a tiny step, maybe 0.2 mm wide and deep ledge.
    Then plunge in z direction cutting only the ledge, with a suitable different tool.

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    Quote Originally Posted by Bill in PA View Post
    This.

    What we used to do was grind and super-finish the primary bevel. We would hard-turn the secondary (if it had one) to maintain the primary bevel width (usually 1 mm).

    Looking at the grind on the reverse bevel, you do not want the ground finish going across the slitter. The grind direction should go radially. If this is a printing or paper converting slitter, this type of grind causes more dust and does not wear as well. The bottom knife is what turns the top slitter. The top slitter is canted at an angle and that type of grind does not work as well as one that is radial.

    If you are going to continue to try hard turning, make sure you are turning starting at the cutting edge and cutting toward the heel of the bevel. This will minimize the burr. You will still have to hone the edge. Depth of cut should be around .005" with maybe a finish pass at .002".

    We would sometimes hard-turn bottom knives depending on application. You could get a good edge since the bevel was only three degrees off vertical. We would ALWAYS grind and super-finish the top knives. If the bottom knives were carbide, then they were super-finished after grinding.

    You are not going to get the same edge quality by turning that you will get with proper grinding and super-finishing.

    Bill

    Thanks for this Bill, very helpful. This is pretty much what we are doing now, we are hard turning them and them finishing them on the grinder. Do you think the rad of the tool is too large we are using 0.8? We will try coming down the bevel rather than going up, and use your suggested cutting depth.

    We really want to get to a stage when we can turn it all, rather the grinding them to finish them off. Do you think we can achieve that?

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    Quote Originally Posted by spock View Post
    looks like some carpet knives I used to make for the automotive industry. I used a .032 radius ceramic insert for the final pass, dont remember the speed/feed, been years since I made them. I bet I was on the slow side, 300 sfm and .003 per rev would be my first guess. Sounds slow, but speed means heat and heat means you ruin the edge. I do remember that last, I used a fine, flat stone to clean up the edge, by holding it flat to the angle as it turned. They were sharp enough to cut you easily.
    I have done a lot of hard turning trim tools for plastic work and I have found that while you will not end up with as good a quality edge as grinding, you can turn/stone a working edge if you have a flat surface to rest your stone on so that you do not roll the edge.

    Thanks for this, yes this is pretty much what we are doing, but would like to do it all via turning rather then honing/grinding.

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    Quote Originally Posted by hanermo View Post
    Just an idea.

    Maybe turn the final cut to almost full radius, leaving a tiny step, maybe 0.2 mm wide and deep ledge.
    Then plunge in z direction cutting only the ledge, with a suitable different tool.
    Humm, thanks. I'll give this a try as well.

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    Warning, I have no experience playing with knives other than sharpening a few for metal shears, so take what I say with a grain of salt if need be.

    I would guess the less than perfect edge is likely not a result of speeds and feeds but rather the last little bit flexing away from the cutter because it is so thin and causing rubbing not cutting. That said here is how I would tackle that job if it were awarded to me.

    Make a fixture that supports the back of the edge. It could either bolt/clamp to the part and have reliefs for the chuck jaws to grab the part or you could hold the fixture in the chuck and clamp the parts to is with a bolt through the center. You will likely need 2 of these fixtures, one to back the un-turned side and another to back the finished side when you flip them. I would face out of the part into the fixture rather than into the part so that you have already established tool pressure and cutting action before reaching the edge. (same idea a climb milling vs conventional)

    If that didn't work I would be getting ready to set up the grinder....

    Again not necessarily the right way to do that job but that is what I see when looking at it.

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    Love it when a person asks a question, and people with experience say "how", and the OP says but we want to do it our way, which doesn't work....

    What material are you Machining? What hardness is it?

    IME you cannot Machine a knife's cutting edge. People been honing knife edges for a Million years, you are not the first person that didn't want to.

    R

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    Quote Originally Posted by litlerob1 View Post
    Love it when a person asks a question, and people with experience say "how", and the OP says but we want to do it our way, which doesn't work....

    What material are you Machining? What hardness is it?

    IME you cannot Machine a knife's cutting edge. People been honing knife edges for a Million years, you are not the first person that didn't want to.

    R
    Sorry but we know it can be done, I've seen it done with my own eyes at a local tooling shop. Machining from blank to form a perfect sharp knife with a great edge all on the CNC lathe. However they wont tell us how's its done in terms of all the variables.

    D2 material, around 60 HRC.

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    Quote Originally Posted by spock View Post
    looks like some carpet knives I used to make for the automotive industry. I used a .032 radius ceramic insert for the final pass, dont remember the speed/feed, been years since I made them. I bet I was on the slow side, 300 sfm and .003 per rev would be my first guess. Sounds slow, but speed means heat and heat means you ruin the edge. I do remember that last, I used a fine, flat stone to clean up the edge, by holding it flat to the angle as it turned. They were sharp enough to cut you easily.
    I have done a lot of hard turning trim tools for plastic work and I have found that while you will not end up with as good a quality edge as grinding, you can turn/stone a working edge if you have a flat surface to rest your stone on so that you do not roll the edge.

    This mostly. ^^^^

    I do some for a building steel maker and hard turn the larger ones. I use .002" feed/.05mm at a speed that gives an orange continuous chip. I use ceramic and feed from the OD to the ID and stone the faces at the same speed.

    Ed.

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    In the seafood industry round knives are sharpened by a fixture holding a round stone against the edge that is rotated by the friction of the blade turning against it. industrial meat slicers have the fixture built into them so they can be sharpened without removing.

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    I've turned a few rotary shear blades to sharp. I'd typically turn them the conventional way (using the tool tip) until the nicks were machined away. That would leave a burr on one side or the other. Then I'd use a black ceramic insert broadside to just kiss each side of the angle. I'd set the tool so it had the tiniest amount of relief clearance towards the secondary relief. Of course, you can't machine this sharp edge to any great width because you'd begin to get tool chatter if you did. I'd maybe hone the edge a little bit afterwards, but only to remove a trace of wire edge.

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    Quote Originally Posted by atex57 View Post
    This mostly. ^^^^

    I do some for a building steel maker and hard turn the larger ones. I use .002" feed/.05mm at a speed that gives an orange continuous chip. I use ceramic and feed from the OD to the ID and stone the faces at the same speed.

    Ed.
    Thanks. How do you stone them on the CNC lathe? I'm assuming its not a CNC lathe you are using, as you cant get in there (cant open door) while its turning to get a stone on there?

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    Quote Originally Posted by mfracing View Post
    Sorry but we know it can be done, I've seen it done with my own eyes at a local tooling shop. Machining from blank to form a perfect sharp knife with a great edge all on the CNC lathe. However they wont tell us how's its done in terms of all the variables.

    D2 material, around 60 HRC.
    Then it was a shit edge. Did you take a microscope to the edge? I guarantee that edge lasted much much less than a proper ground/stoned/lapped edge. What you saw was no where near "perfect".

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    Quote Originally Posted by HuFlungDung View Post
    I've turned a few rotary shear blades to sharp. I'd typically turn them the conventional way (using the tool tip) until the nicks were machined away. That would leave a burr on one side or the other. Then I'd use a black ceramic insert broadside to just kiss each side of the angle. I'd set the tool so it had the tiniest amount of relief clearance towards the secondary relief. Of course, you can't machine this sharp edge to any great width because you'd begin to get tool chatter if you did. I'd maybe hone the edge a little bit afterwards, but only to remove a trace of wire edge.
    Thanks, but how do you hone it on a CNC lathe with door?

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    Quote Originally Posted by Rewt View Post
    Then it was a shit edge. Did you take a microscope to the edge? I guarantee that edge lasted much much less than a proper ground/stoned/lapped edge. What you saw was no where near "perfect".
    Not under a microscope, but plenty good enough to cut the material they are designed to cut. Looks prefect to the eye, and much better then what we are getting in those pics!

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    A knife edge is not what most people think it is. Micro serrations is what makes the best cutting edge.

    Not sure if the OP understands the fundamentals of runout, but .0002" runout is going to a real problem. And I'm not sure D2 is best, maybe 440c??


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